Transparencies are used in a variety of different applications including vehicular applications such as in marine, land, air and/or space vehicles and in non-vehicular applications such as in buildings and other stationary structures. In vehicular applications such as in commercial aircraft, transparencies may be mounted along the aircraft cabin and around the aircraft flight deck and may include windshields and other forward, side and overhead windows. Transparencies may be formed of glass and polymeric materials or as laminated combinations of glass and polymeric materials. Polymeric materials for transparencies may include, without limitation, acrylic and polycarbonate compositions.
When fabricating a transparency of polycarbonate material, certain optical defects may occur during the forming process. For example, carbon particulates may occur during the formation of a polycarbonate transparency and may appear as relatively small black spots that are embedded within the transparency. When viewed through the transparency, an embedded carbon particulate may be misinterpreted as a long-distance object.
Included in the prior art are several methods for inspecting transparencies for optical defects. For example, certain aircraft transparencies such as an aircraft canopy may be manually inspected by looking upwardly though the canopy searching for defects by using the sky as a background to backlight the transparency. This inspection technique requires generally clear (e.g., non-cloudy) atmospheric conditions in order to provide a homogenously lit background against which an inspector can view the entirety of the transparency. As may be expected, this inspection technique can result in significant aircraft downtime while waiting for the appropriate atmospheric conditions.
Although camera-driven methods have been developed in the automotive industry for automating inspection of transparencies, such automated camera methods may lack the resolution required for aerospace transparencies. For example, inspection methods used in the automotive industry are typically directed toward high-speed inspection on a production line wherein the size of allowable defects in the automotive transparency is typically larger than the allowable defect size (e.g., 0.030 inch) of aerospace transparencies. In this regard, the resolution at which an automotive transparency is inspected is sacrificed in the interest of high-volume production.
Furthermore, inspection methods used in the automotive industry are typically directed toward transparencies having relatively slight curvatures as compared to aircraft transparencies such as aircraft canopies and windshields which may have more complex curves that may be of smaller radius. In addition, the cross-sectional layup of an aircraft transparency such as an aircraft windshield is generally more complex than an automotive transparency due to the higher strength requirements and increased thickness (e.g., up to 1 inch thick or larger) of an aircraft windshield as required for surviving bird strikes and handling structural loads.
As can be seen, there exists a need in the art for a method for accurate detection of defects of relatively small size (e.g., approximately 0.010 inch or smaller). Additionally, there exists a need in the art for a method for detecting optical defects in a transparency in a rapid manner in order to reduce inspection time. Furthermore, there exists a need in the art for a method for detecting optical defects in a transparency that provides an automated means for documenting the size and location of optical defects in order to characterize the source of the defect. The need to accurately quantify an optical defect (e.g., measure the defect size and document the location) in an aircraft transparency is desirable due to the relatively high cost of replacing an aircraft windshield as compared to the cost of replacing an automotive windshield.